ok, first attempt using sketchup 8

[Jonathan]

Once you include torsional stiffness in the calculation the single box section is much better than RSJ:



The problem with either of these options is mounting the rails with sufficient accuracy. I doubt either steel RHS or RSJ will we sufficently accurate to mount the two rails on opposite faces, since the variation in the 'height' of the section will change the spacing of the rails leading to binding. Seems a bit risky... could fix it with shims I guess but you don't want to go there.
Instead I'd use two pieces of the 100x60 box section with one rail on each as that will be far stronger than just one and solves the problem of maintaining rail spacing.

[JAZZCNC]

The problem with either of these options is mounting the rails with sufficient accuracy. I doubt either steel RHS or RSJ will we sufficently accurate to mount the two rails on opposite faces, since the variation in the 'height' of the section will change the spacing of the rails leading to binding. Seems a bit risky... could fix it with shims I guess but you don't want to go there.
Instead I'd use two pieces of the 100x60 box section with one rail on each as that will be far stronger than just one and solves the problem of maintaining rail spacing.

Rubbish.!! What makes you think both these will be flat surface or straight .? Single or 2 pieces make no difference the surface will still need some work and while 2 pieces will allow easy general positioning they won't be anywhere near parallel all along the length. If the section has any bend then it will still need grinding or shimming at several points and just because you have a rail on each piece doesn't make it any easier.
It's not difficult with a decent straight edge and files,grinder to the bring the main reference surface upto scratch then it's just a case of shimming or grinding the other rail surface parallel.

[deannos]

Ok, here is my latest attempt, trying to incorporate every thing that has been suggested. So i will be using 20mm supported rails on every axis and 1610 ballscrews on every axis, along with belts and pulleys. The 2 rsj's will be 178mmx102mmx19mm and the gantry is 100mmx60mmx3mm steel box section. The gantry end plates and bearing plates are 20mm steel. The Z axis will be using 20mm ali plate. I have spread the gantry bearings out as been advised. Hopefully the drawings with dimensions will help.


The rails are 300mm,800mm and 1400mm. The screws are 350mm,800mm and 1450mm long

Also now confused after reading comments on the other post about the "effect of forces on gantry cross sections" and the following was mentioned-

I don't like box section for gantry's has it can produce resonance and vibration unless thick walled.? The bolted together extrusions are far denser therefore absorb vibrations far better.!! . . .Any resonance will transfer thru to the finish.

Am i going round in circles

[JAZZCNC]

Ok thats good but bring the screw to the front, will be much stronger having the ball-nut close the Z Axis back plate.

Have you got the 20mm steel plate.? If not I think you'll maybe change your mind when you see the price.!! It's not really needed either and I'd use ali for easy working.

[deannos]

Ok thats good but bring the screw to the front, will be much stronger having the ball-nut close the Z Axis back plate.

Ok, thats no problem, i will do that. Is this to prevent racking etc

Have you got the 20mm steel plate.? If not I think you'll maybe change your mind when you see the price.!! It's not really needed either and I'd use ali for easy working.

No i haven't got any steel plate and a quick google has made me change my mind, but if i use ali how would I join this to the steel gantry?

[JAZZCNC]

No i haven't got any steel plate and a quick google has made me change my mind, but if i use ali how would I join this to the steel gantry?

Bolt or weld Angle brackets to box then bolt thru Ali plate into brackets.

[deannos]

Bolt or weld Angle brackets to box then bolt thru Ali plate into brackets.

Excellent, an easy solution i didn't even think of.
cheers Dean

[deannos]

Went to pick up my cheap box section 100mm x 60mm and it was bent, so left it. But got 2 pieces of RSJ 160mm x 90mm x 7mm x 2000mm for £20 on ebay and local for pick up. I've done a few more workings on my last design, but as there were no replies, it's probably no good. Am i going backwards in my design, i have an idea in covering the 2 outside ball screws using tumble dryer venting duct and curtain wire, it might work.

All the red/brown areas are 40mm x 40mm steel box section, that is reasonably priced. All the ali is 20mm except for the motor mounts where i have used 10mm, milled to 3mm for clearance.




The screw lengths and rail lengths are still as quoted a couple of posts ago. I have at the moment got 1610 on the Z, reading through some other posts it seems i would be better with 1605 on Z, is that correct. Also I'm looking at going with 1:1 ratio with pulleys, would that be OK.

If i have made a big mistake with this design, please let me know. If you want the file to check it out, I'm willing to send it. I'm itching to place an order with chai.

[WandrinAndy]

I'm itching to place an order with chai.

That makes at least two of us Dean! I'm finding that the tweaking, particularly around aligning multiple pulleys and belts, is taking quite long but reckon it's worthwhile getting this right up front in the virtual world.

Btw, your design is looking great to this newbie. It amazes me how many design permutations there are... Suppose it's a good thing as it provides more ideas for us all to crib!

[JAZZCNC]

Also now confused after reading comments on the other post about the "effect of forces on gantry cross sections" and the following was mentioned-

Am i going round in circles

Dean don't worry about it your only cutting wood the box section will easily handle it. The box can resonate but cutting wood it won't really matter or show.
There are also ways around this if it does by filling with sand. So long has the resonance doesn't affect the motors then you won't have problem and because your using belts then this won't happen and why it's good idea to use belts.